Breast cancer is the most common cancer diagnosis in women and is also one of the leading causes of cancer-related mortality in women who suffer from this condition. Tumors that originate in the breast consist of heterogeneous populations of cells. Breast cancer stem cells (BSCSs) are a subtype of tumor cells that have properties similar to normal, tissue stem cells such as the ability to divide slowly and give rise to differentiated cellular lineages. Furthermore, BCSCs have been implicated in tumor initiation, therapy resistance and metastasis to distant organs. Given this information, a greater understanding of the biological processes that sustain BCSCs will lead to the development of novel agents directed against this chemo- and radio-resistant population of tumor cells. The proposed work will explore the role of the ?6 integrin splicing variants, ?6A?1 and ?6B?1, in the genesis of BCSCs, specifically addressing the mechanism of activation of the TAZ transcriptional coactivator. Integrins are a family of cell surface receptors that function in signal transduction and adhesion to the extracellular matrix. The ?6A?1 integrin variant is expressed in differentiated, epithelial breast cancer cells and inhibits the acquisition of stem cell properties Conversely, the ?6B?1 integrin variant is expressed in BCSCs and promotes tumor initiation by activating the Hippo signaling pathway transducer TAZ. TAZ has previously been shown to be important in the functioning of BCSCs, but the mechanism is unknown. Therefore, elucidating the relationship between the ?6 integrin splicing variants and TAZ activation will provide insight into mechanisms of breast cancer progression. This proposal will use a cellular and molecular biology approach to establish the mechanism by which TAZ is suppressed in the ?6A?1 expressing non-stem breast cancer cell population (Aim 1). Biochemical studies will also be undertaken with the purpose of connecting mechanisms of TAZ inactivation with classical Hippo pathway signaling in the non-stem breast cancer cell population (Aim 2). In summary, the studies included in this proposal will increase the understanding of integrin regulation of BCSCs. Our results can provide rationale in designing future targeted therapies for treatment resistant subtypes of breast cancer.